Water, Earth, Air and Fire. It's curious that Milton refers to water as Sea, and earth as Shore. The idea of the Four Elements originated with the Greek philosopher, Empedocles. They are in continual conflict with each other as a result of Strife. See http://www.webwinds.com/myth/elemental.htm

According to Empedocles, an early Greek philosopher, there are four elements in continual conflict with each, as influenced by Strife (discordant energy) and Love (harmony). See my article on Empedocles and the four elements at: http://www.webwinds.com/myth/elemental.htm